Conventional electromagnetic clutch assemblies often utilize opposing clutch surfaces or toothed faces which are mounted for coaxial rotation, with electromagnetic control of engagement and/or disengagement to transmit rotational driving forces therebetween. Such electromagnetic devices normally include an axially movable armature element which is connected to an axially retained prime mover to transmit torque. The elements either are resiliently or slidably movable under the control of an electromagnetic field to engage or disengage the opposing clutch elements.
One application of such eletromagnetic clutch assemblies is in aircraft electrical systems which supply electrical needs of the craft and include one or more electrical generators which are driven either directly from the aircraft engine or through some form of speed reducing gear box. In such aircraft applications, the generators may operate at high speeds which can be in excess of 12,000 RPM. For instance, the clutch assembly may be engageable and disengageable to connect and disconnect a starter generator with a torque converter. Therefore, long life and minimum wear are essential in the design and construction of the clutch assembly because of repeated engagements. In addition, the size of the clutch assembly has become of considerable importance in arriving at compact drive systems.
One of the problems in designing an electromagnetic clutch assembly having long life, minimum wear and compact dimensions revolves around the different characteristics of the components of the clutch. The opposing clutch elements, whether friction surfaces or toothed faces, must be fabricated of a material which is sufficiently ductile to have high impact resistance. On the other hand, the armature for the electromagnetic device must be capable of high level flux saturation. These contradicting requirements often result in compromises or in complex and oversized clutch design.
In other words, an alloy steel, such as vanadium permendur which is a cobalt alloy steel, can be heat treated on the order of 1,380.degree. F. in order to obtain a high level of flux saturation, but such a heat treatment would lose the mechanical properties of the metal, such as high impact resistance. On the other hand, should the metal be heat treated on the order of 980.degree. F. in order to obtain optimum strength properties for the metal, the fabricated component would not be a sufficiently good conductor for the magnetic flux. High strength or high impact resistance must be accomplished in the clutch elements themselves for operation at high speeds.
Because of the differing requirements for a clutch element of a high speed clutch assembly and the armature for such an assembly, heretofore, either compromises had to be made or the clutch design became complex and unduly large. Clutch assemblies wherein the magnetic flux path passed through the clutch elements themselves simply have not been applicable for high speed aircraft applications. This invention is directed to solving the above problems by providing an electromagnetic clutch assembly of a novel design to provide for long life, minimum wear, repeated engagement and compact size.